The present invention relates to the manufacture of semiconductor integrated circuits. More particularly, the present invention relates to improved techniques for performing a nitride (Si.sub.3 N.sub.4) etch in a variable-gap plasma processing system, which advantageously improves substrate throughput while minimizing particulate defect density.
In the fabrication of semiconductor devices, e.g, semiconductor integrated circuits (ICs) or flat panel displays, devices such as component transistors are typically formed on a substrate, e.g. a silicon wafer or a glass panel. The etching of a nitride, or Si.sub.3 N.sub.4 layer, is commonly performed in the manufacture of certain integrated circuit devices such as complementary metal oxide semiconductor (CMOS) transistors. Nitride layer etch, also known as well nitride etch or tank nitride etch in the case of CMOS transistors, is typically performed to define the n and p wells, for example.
To facilitate discussion, FIG. 1 depicts a simplified layer stack 100, representing the layers that may be formed above a semiconductor substrate during semiconductor IC fabrication. In FIG. 1 as well as the figures herein, it should be noted that the layers shown therein are illustrated only; other additional layers above, below, or between the layers shown may be present. Further, not all of the shown layers need necessarily be present and some or all may be substituted by other different layers using knowledge commonly possessed by those skilled in the art.
Layer stack 100 generally includes a substrate 102, which is typically formed of silicon. Above substrate 102, there may be disposed an oxide layer (SiO.sub.2) layer 104. A nitride (Si.sub.3 N.sub.4) layer 106 is shown disposed above oxide layer 104. To etch a desired pattern in nitride layer 106, an overlaying photoresist (PR) layer 108 is then formed atop the blanket deposited nitride layer 106. Photoresist layer 108 may then be patterned (e.g., through a conventional photoresist technique) to facilitate the etching of the underlaying nitride layer 106. By way of example, one such photoresist technique involves the patterning of photoresist layer 108 by exposing the photoresist material in a contact or stepper lithography system, and the development of the photoresist material to form a mask to facilitate subsequent etching. Using an appropriate etchant, the areas of nitride layer 106 that are unprotected by the photoresist mask are then etched away, leaving behind a desired pattern on nitride layer 106.
In the prior art, the etchant employed to etch through nitride layer 106 is typically a mixture comprising SF.sub.6 and helium. When excited into a plasma (e.g., by radio frequency or RF energy), the fluorine species of the plasma etch through the unprotected areas of nitride layer 106 to form silicon fluoride, which is then evacuated away. The helium component in the prior art SF.sub.6 /helium chemistry is employed typically to assist in the distribution of the plasma etchant throughout the substrate, thereby improving uniformity. Further, helium may also help in cooling the substrate during etching in order to, for example, prevent the protective photoresist features from burning up.
It has been found, however, that the use of prior art SF.sub.6 /helium chemistry for etching nitride layer 106 typically requires a fairly narrow gap between the top surface of the substrate and the top electrode of the plasma processing chamber. When the prior art SF.sub.6 /helium chemistry is employed, the narrow gap is required to ensure an acceptable etch result. However, the requirement of a narrow gap has several disadvantages.
To facilitate discussion, FIG. 2 depicts a typical plasma processing chamber 200, representing a plasma processing chamber typically employed in the prior art to etch through the nitride layer. In the present example, plasma processing chamber 200 represents a plasma processing chamber of a plasma processing system known by the brandname of RAINBOW 4400.TM., which is available from Lam Research Corporation of Fremont, Calif. Although the RAINBOW 4400.TM. is employed herein to facilitate discussion, it should be borne in mind that the technique disclosed herein is not limited to this particular configuration; the inventive and disclosed nitride etch technique may be adapted, using knowledge commonly possessed by those skilled in the art, to other plasma processing chamber configurations.
Plasma processing chamber 200 typically includes a lower electrode or chuck 202, which is typically grounded. Substrate 204, representing a substrate having thereon a nitride layer to be etched, is typically disposed above lower electrode 202 during etching.
An upper electrode 206 is disposed above substrate 204 and is separated therefrom by a gap 208. Upper electrode 206 is mounted to a movable backing plate 210, typically in the form of a large circular metal disk. Movable backing plate 210 and upper electrode 206 may be moved along the direction of the z axis by a gap drive assembly which includes a plurality of lead screws 212, a chain 214, and a gap drive motor 216. By changing the direction of rotation of gap drive motor 216, movable backing plate 210 and upper electrode 206 may be moved toward or away from electrode 202, thereby varying the size of gap 208.
During the etch, the pressure within plasma processing chamber 202 is typically maintained at a lower pressure than the ambient environment pressure. In one embodiment, the nitride etch is carried out at a chamber pressure of about 500 milliTorr (mTorr). To maintain the pressure differential between the interior of plasma processing chamber 200 and the ambient pressure, seals 220 are typically provided around the periphery of movable backing plate 210. Seals 220, of which there are two in FIG. 2, are typically formed of a relatively non-reactive sealing material such as a suitable rubber, e.g., VITON.TM. rubber. To reduce friction between seals 220 and the interior surface of chamber wall 224 as backing plate 210 is moved toward or away from the substrate, seals 220 are typically lubricated with a suitable lubricant.
To facilitate etching, an etchant source gas rapture, e.g., SF.sub.6 /helium in the case of the prior art nitride etch, is typically flowed into chamber interior 226. In the configuration of FIG. 2, upper electrode 206 has a showerhead configuration, i.e., upper electrode 206 is provided with a plurality of apertures for releasing etchant source gases into chamber interior 226. However, the etchant source gases may also be provided through other mechanisms, e.g., via apertures in chamber wall 224 or a gas ring surrounding lower electrode 202.
An RF power source 228 is then turned on to provide RF energy to upper electrode 206. RF power source 228 is typically coupled to upper electrode 206 via an RF timing network 230 of a conventional design. RF tuning network 230 functions to minimize the impedance between RF power source 228 and plasma processing chamber 200, thereby maximizing power delivery. The supplied RF power ignites or strikes the plasma from the supplied etchant source gases within chamber interior 226 to etch the unprotected areas of the nitride layer. Reaction byproduct gasses are then exhausted away through an exhaust port 240. Exhaust port 240 may be coupled to an automatic pressure control (APC) system 242, which automatically varies the rate of the gas exhausted through exhaust port 240 to maintain the desired chamber interior pressure.
As mentioned earlier, the prior art SF.sub.6 /helium chemistry, which is employed to etch the nitride layer, typically requires a fairly narrow gap 208, e.g., between 0.8 cm to 1.2 cm, to achieve an acceptable etch result. This gap clearance is typically insufficient to ensure proper loading and unloading of substrate 204. By way of example the robotic arm that is typically employed to move substrate 204 from load lock 244 into chamber interior 226 and to position substrate 204 on a lower electrode 202 typically requires a gap clearance greater than the aforementioned gap distance of 0.8 cm to 1.2 cm. Likewise, when etching is completed and substrate 204 is lifted off lower electrode 202 (employing for example, lifter pin 246), a gap clearance greater than the abovementioned 0.8 cm to 1.2 cm must be provided to permit the robot arm to move the etched substrate from chamber interior 226 into load lock 244.
To provide the required gap clearance for proper loading and unloading, the prior art nitride etch technique requires that movable backing plate 210 (and upper electrode 206) be moved away from lower electrode 202 during the loading of substrate 204. Gap drive motor 216 then engages to lower movable backing plate 210 (and upper electrode 206) toward substrate 204, thereby maintaining a proper gap 208 between the lower surface of upper electrode 206 and the upper surface of substrate 204 to facilitate nitride etching. When the etch is completed, gap drive motor 216 then engages to raise movable backing plate 210 (and upper electrode 206) away from substrate 204 to facilitate the unloading of the substrate from chamber interior 226.
Through experience, it has been found, however, that the moving of movable backing plate 210 creates many disadvantages. By way of example, because of the pressure differential between chamber interior 226 and the ambient pressure, a large amount of stress is typically imposed on the gap drive assembly (e.g., on lead screws 212, chain 214, and/or gap drive motor 216), whenever gap drive motor 216 is engaged to move movable backing plate 210 toward or away from substrate 204. Accordingly, the gap drive assembly has been found to be susceptible to a high degree of wear and frequent failures, necessitating the temporary cessation of the etching operation for maintenance and/or replacement. The high frequency of maintenance and/or repair reduces the throughput of the plasma processing system, i.e., reduces the number of substrates that can be etched over a given period of time, thereby increasing the plasma processing system's overall cost of ownership.
Further, it has been found that rubber seals 220 degrade over time as they are moved. When seals 220 wear away, some of the seal material, e.g., rubber particles, ay be introduced into chamber interior 226, thereby increasing the level of particulate contaminants within chamber interior 226, and the defect density in the resulting semiconductor device. Further, worn seals may cause atmospheric leaks, which introduce unwanted ambient air into chamber interior 226 during the etch process, leading to etch defects.
Still further, the seal lubrication material employed to reduce friction between seals 220 and chamber wall 224 when movable backing plate 210 slides along the chamber wall may age over time, causing lubrication particles to flake off into chamber interior 226, further increasing the level of particulate contamination therein and increasing the defect density in the etched semiconductor devices.
Some modern fixed gap plasma processing systems, e.g., TCP.TM. brand, systems also available from the aforementioned Lam Research Corporation, do not have a movable upper electrode and consequently do not suffer the aforementioned gap-drive related contamination and maintenance problems. Despite the disadvantages associated with nitride etching in variable-gap plasma processing systems, some semiconductor manufacturer have nevertheless found themselves in a situation wherein a large amount of capital has already been expended to acquire variable-gap plasma processing systems. The capital investment in variable-gap plasma processing systems requires that the use of variable-gap plasma processing systems continue to justify their acquisition costs, at least until the acquisition costs are recouped.
To reduce the frequency of maintenance and/or repair as well as the possibility of unwanted particulate contamination within chamber interior 226, there have been attempts at formulating a nitride etch process wherein gap 208 can be fixed (i.e., movable backing plate 210 does not have to be moved) during the loading of substrate 204, the plasma etching of the nitride layer disposed thereon, and the unloading of substrate 204 after etching. However, these attempts have largely been unsatisfactory.
In particular, it has been found that most chemistries commonly employed for performing the nitride etch do not yield satisfactory etch results when gap 208 is fixed at a gap distance suitable for loading and unloading of substrate 204. Further, when gap 208 is enlarged, the volume of chamber interior 226 correspondingly increases, which complicates pressure stability issues during etching. By way of example, it has been found that the large volume of chamber interior 226, which is caused by increasing gap 208, makes it difficult and time-consuming to stabilize the pressure within chamber interior 226 prior to etching.
Stabilization refers to the step taken prior to etching to ensure that the desired process parameters are stable. In general, the processed parameters such as pressure, etchant gas flow rate, temperature, and the like within chamber interior 226, must be substantially stabilized before etching can begin. This is because the values of these parameters within chamber interior 226 may fluctuate initially, e.g., when etchant source gasses are initially flowed into chamber interior 226 and RF power source 228 is initially turned on. If etching is conducted while the process parameters fluctuate, the etch results, e.g., uniformity, etch rate, selectivity, and the like, may be unpredictable and/or other than desired.
As can be appreciated by those skilled in the art, the duration of the stability step is preferably minimized to the maximum extent possible since no etching occurs during the stability step. Longer stability steps tend to reduce the substrate throughput, which lowers productivity and increases the cost of ownership of the plasma processing system.
It has been found, however, that with a larger gap dimension, prior art techniques of stabilizing the pressure chamber interior 226 typically result in an unacceptably time-consuming stability step. To facilitate discussion, FIG. 3 depicts two lines, 302 and 304 on a pressure versus time plot. Line 302 depicts the pressure within chamber interior 226 during a typical nitride etch process when the gap is relatively small, e.g., 1.2 cm in the case of the prior art SF.sub.6 /Helium chemistry. On the other hand, line 304 depicts the pressure within chamber interior 226 for a nitride etch process when gap 208 is increased.
At point 306 on the time scale, the stability step begins. At this point, the prior art typically stabilizes the pressure within chamber interior 226 at the process pressure set point, i.e., the pressure at which nitride etching is conducted (which may be 500 mT in one prior art example). As mentioned earlier, pressure within chamber interior 226 is regulated by withdrawing an appropriate amount of gas through exhaust port 240 (using automatic pressure control system 242).
At point 308, RF power source 228 is turned on. The introduction of RF power excites the gas within chamber interior 226, causing the pressure therein to increase. Pressure increases to point 310, causing automatic pressure control system 242 to engage to compensate and bring pressure down to the process pressure set point of 500 mT. The pressure within chamber interior 226 stabilizes at the process pressure set point of 500 mT at point 312 in FIG. 3. Once stabilized, the etch may begin. At point 314, the etch is completed.
When gap 208 is increased, the larger volume of chamber interior 226 causes a greater amount of pressure increase when RF power supply 228 is turned on. Again, automatic pressure control system 242 engages at point 316 to compensate. However, the greater amount of pressure variation in chamber interior 226 due to a larger gap 208 lengthens the stability step. The pressure within chamber interior 226 does not stabilize at the process pressure set one of 500 milliTorr until point 318. Because the etch process does not begin until pressure is stabilized at the process pressure set point of 500 miiliTorrs, etching begins later with the etch process associated with line 304, compared with the etch process associated with line 302 (at point 318 which is later in time than point 312).
Because etching did not begin until point 318 on line 304, which is later in time than point 312 on line 302, it did not end until point 320 on line 304, which is later in time than point 314 on line 302. Accordingly, the etch process associated with a larger gap 208 takes longer from the moment the stabilize step begins until the time the etch ends (from point 306 to point 320 on the timeline). As shown in FIG. 3, this duration is clearly longer than the etch process associated with a narrow gap (associated with line 302), which begins at point 306 and ends at point 314.
To some semiconductor manufacturers, the increase in the time required for a nitride etch cycle due to an enlarged gap distance results in an unacceptably low substrate throughput (i.e., productivity), and an unacceptably high cost of ownership. Because of these hurdles, prior art attempts at fixed-gap nitride etching in variable-gap plasma processing systems, particularly when the prior art technique of stabilizing the process pressure is employed, has been found to be relatively unproductive.
In view of the foregoing, there are desired improved techniques for etching the nitride layer of a semiconductor substrate in a variable-gap plasma processing system The improved technique preferably permits the use of a fixed gap during the loading, etching, and unloading of the substrate in order to reduce the frequency of maintenance and/or repair, and the level of particulate contamination within the variable-gap chamber interior.